By Jon Schuppe, NBC News

NBC News has projected that President Barack Obama will win a second term after he racked up a string of victories in crucial battleground states.

“This happened because of you. Thank you,” Obama tweeted at 11:14 p.m. ET.

One after another, crucial states that had been deemed toss-ups before Election Day fell into the president’s hands: Pennsylvania, Wisconsin, New Hampshire, Iowa. With each Obama win, the path to victory for Romney got narrower and more demanding.

Finally, the results in Ohio put Obama over the top, handing him the Electoral College swing votes he needed to remain at the reins of a divided and anxious nation.

So many people turned out to vote Tuesday that several crucial states, including Florida and Virginia, remained open after closing to accommodate the people who waited in long lines that snaked from the doors of polling places.

Obama, the country’s first black president who won election in 2008 on a promise of hope and change, triumphed this time with a starkly different message: asking voters to stick with him as he continues trying to fix the economy and improve America’s standing in the world.

He survived a long, punishing, exorbitant race against former Massachusetts Gov. Mitt Romney that highlighted two contrasting visions of the country. Where Romney emphasized the need to lower taxes, relax federal regulations and cut government spending, Obama promised to raise taxes on the wealthiest Americans and deploy government’s help in pulling the country out of the economic doldrums.

Obama’s win also means that his signature piece of legislation, the over haul of the country’s health-care system, will remain law.

The president’s re-election, along with what appears to be no change in the balance of power Congress, means that, in broad terms, nothing that much will change in Washington.

Polls have been predicting for weeks that the race would turn out to be a nail-biter.

After dueling for months over the airwaves, on the stump and in person, Obama and Romney could do little more than watch the returns as they arrived, piecemeal, into their campaign headquarters Tuesday night.

Obama settled into his hometown of Chicago, making last calls to voters, enjoying a customary game of pickup basketball and having dinner with his family. Romney appeared in last-ditch rallies in Ohio before heading home to Boston.

But with the outcomes in the handful of swing states still uncertain, anxiety remained high.

Picking up Wisconsin and Pennsylvania is not only important for Obama’s hope for re-election, but also symbolic. Wisconsin is the home state of Romney’s running mate, Rep. Paul Ryan. And Pennsylvania was subject of a last-minute surge of campaigning by Romney.

The most crucial ballots are being cast in a small number of swing states whose votes in the all-or-nothing Electoral College could still go either way – as both candidates try to reach the magic number of 270 electoral votes.

• The biggest step in the path toward the White House is Florida and its bounty of 29 electoral votes. Both Obama and Romney believe they have a shot at winning the state, which was ground zero in the infamous 2000 showdown between George W. Bush and Al Gore. The state was awarded to Bush that year, and in 2004, but went to Obama in 2008. This time around, Romney has led most polls, and it is considered a must-win for him on Election Day. If Obama takes Florida, then the odds tip heavily in the president’s favor, leaving Romney having to win all the other battleground states in order to eke out a victory. It is no wonder, therefore, that lawyers for the campaigns were girding for a fight in the courts over polling procedures.

• Ohio, with 18 electoral votes, is the nation’s key bellwether state, and is as essential to Obama as Florida is to Romney. The president enjoys a narrow lead in the polls in Ohio, largely because most of the state benefited from his bailout of the auto industry. The Buckeye State is known as Obama’s “firewall,” because winning it would make Romney’s path to 270 extremely difficult. If Romney wins Ohio, then his odds improve significantly. A key statistic to bear mind about Ohio: it has sided with the winning candidate in every presidential election since 1960.

• Virginia’s prize is 13 electoral votes, and although it has gone to the Democratic presidential candidate just twice in the past sixty-plus years, it has leaned toward Obama this year.

• Colorado promises nine electoral votes for the candidate who wins it, and pre-election polls gave a slight edge to Obama, even though the state has historically been a red one. It has become a more competitive state in large part because of its growing Hispanic population. That was evident in 2008, when Obama became the first Democratic presidential candidate to win the state since the 1960s.

• Iowa, which offers six electoral votes, is another battleground state that has leaned narrowly toward Obama in the days before the election. But it has a habit of swinging back and forth: Obama won it handily in 2008, George W. Bush barely won it in 2004, and Al Gore narrowly won it in 2000.

These states are where Romney and Obama and their Super PAC proxies have focused most of their attention – and nearly $1 billion in advertisements.

So far, NBC News has projected Romney winning, predictably, across swaths of the South, Midwest, and West. Obama has nailed down several northeastern states, along with his home state of Illinois, Michigan and New Mexico.

With so much riding on relatively few votes, the campaigns remained in war mode even after the candidates themselves stopped campaigning. Lawyers on both sides spent the day wrangling over early voting procedures, voter-identification requirements and the counting of provisional and absentee ballots.

Meanwhile, thousands of people are struggling to vote in areas battered by Hurricane Sandy. Long lines and mass frustration in parts of the two states where many people had no place to live, let alone vote. In New York and New Jersey, officials scrambled for backup generators, and many residents were forced to travel to polling places outside of their neighborhoods. New Jersey Gov. Chris Christie made a last-minute decision to allow victims who lost their homes to vote by email.

More than 30 million people, Obama among them, have taken advantage of early-voting laws and cast their ballots ahead of time.

Polls on the East Coast begin closing at 7 p.m., and it could be a long night before the results are known.

The election offers a choice between two starkly different men: the buttoned-up, business-savvy Romney, and the still-popular but battle-scarred Obama.

But it is much more than that. It is also a decision about the role of government in American life; who ought to pay more taxes; the future of entitlement programs like Medicare and Medicaid; and the survival of Obama’s signature domestic legislation, the overhaul of the country’s health-care system.

At its core, the election will reflect who the majority of America believes is better equipped to lead the country out of the economic doldrums, which has pushed record number of people to food stamps and other public assistance, and unemployment to 7.9 percent.

The winner will also have the power to remake the Supreme Court. Several justices have reached retirement age, meaning that whoever takes office in January could get the opportunity to tip the court’s political makeup in his favor.

Control of Congress is also up for grabs on Tuesday. Republicans are trying to maintain their majority in the House of Representatives; Democrats need to win 25 seats to take power. In the Senate, Democrats have a three-vote majority, which means that if Obama wins and gives his party the edge in tiebreakers, the GOP needs to win four additional seats. If Romney wins, then the Republicans only need three seats.

The Congressional results will help set the tone for the next two years in Washington. They will determine how difficult it will be for the president ability to deliver the promises he made on the campaign trail, and lay the groundwork for negotiations over the so-called fiscal cliff, a combination of expiring tax cuts and massive government spending reductions that awaits lawmakers at year’s end.

Obama, 51, was elected in 2008 on a promise of hope and change but was tested from the get-go. He inherited a near-collapse of the economy and national malaise over two lengthy wars. He was unable to rise above the partisan rancor of Washington, and his checkered first term left him bruised and humbled, but also adaptive. He claims that the economy would have been much worse if it wasn’t for the economic stimulus bills he pushed through Congress, and counts among his top achievements a bailout of the American auto industry, the adoption of his health-care reform initiative, the education-reform program called Race to the Top.

“Our work is not yet done,” Obama told a rally in Wisconsin on Monday.

The president ended his campaign with a Monday evening rally in Iowa, where he was accompanied by his wife, Michelle, several longtime friends and advisers, and Bruce Springsteen. He’ll watch returns from his base in Chicago.

Romney, 65, a wealthy venture capitalist who saved the 2002 Winter Olympics in Salt Lake City from insolvency, first ran for president four years ago and never really stopped. His campaign focused on Obama’s shaky economic record, promising to bring a businessman’s experience to the problem.

Just a couple months ago, Romney was seen as headed for a landslide defeat, but he rallied with a dominant performance in the first of three presidential debates. That showing nearly eliminated Obama’s lead, and the race remained tight until Election Day.

Romney capped his campaign with a whirlwind tour of battleground states, ending it in Pittsburgh as part of an 11th-hour push to win Pennsylvania. He planned to return to his headquarters in Boston to await the results.

Everywhere he went, Romney supporters chanted the number 45, a reference to the fact that, if elected, he’d be the 45th president of the United States.

“Tomorrow, we begin a new tomorrow,” Romney said in Florida.